The present invention relates generally to filter presses for separating liquids and solids in sludges, slurries and other feed materials, and more specifically to presetting the thickness and consistency of such material in the wedge zone prior to advancing the material into a higher pressure zone.
Belt filter presses are used to separate liquids (most often water) and solids in sludges, slurries, fiber suspensions, cellulose or other fibrous materials and similar substances. The materials to be dewatered or deliquified could be sewage, industrial waste, paper pulp or any other biological, chemical, mineral-based or industrial material requiring deliquification. The term "sludge" will be used herein to refer to any substance which might require deliquification. The need to deliquify a substance can range from a desire to dry a substance for incineration (so that less fuel is used during the incineration process) to simply removing excess liquid prior to transportation (in order to decrease transportation costs). Since all sludges differ from one another in consistency, viscosity, density, water content and many other parameters, separating the liquids from the solids in sludges is a complex art.
Belt filter presses can employ several stages at which different techniques are used to remove the liquid from a sludge. The use of these different stages improves filtering efficiency throughout the belt filter press. Even prior to commencing liquid removal, a suspension, which might only be one half to one percent solids, can be treated with a polymer or other chemical which coagulates or flocculates the solids. In a typical belt filter press, the flocculated sludge is then moved into a gravity drainage section or stage of the belt filter press where the sludge is stirred and churned so that free water drains off by gravity. At the end of the gravity drainage zone, the sludge could be near 10% solids. The gravity drainage stage is used primarily to remove free water, and to prepare the sludge for, in some cases, a suction stage where additional water is suctioned off and/or a low pressure stage which continues to prepare the sludge for higher pressure stages of deliquification. It is at the higher pressure stages that the sludge is carried between an upper and a lower belt which together follow a path between progressively smaller perforated drums, and finally between progressively smaller solid rollers. While the pressure in the drum area may be considered to be a medium pressure, for purposes of this application, the perforated drum stage will be considered part of the high pressure stage.
As those in the art can appreciate, the pressure imparted to the material between the upper and lower belts in the high pressure stage increases greatly from the largest perforated drum to the smallest solid roller. After completing travel through the high pressure stage, the upper and lower belts separate from one another, and the dried sludge or cake, which can typically be 30% solids, is discharged.
To more efficiently prepare a sludge for the high pressure stage, a wedge zone is often interposed between the gravity drainage stage, suction stages, etc. and the high pressure stage. The result is greater deliquification in the high pressure stage since the wedge zone prepares the sludge for the application of higher pressures. In the wedge zone, the upper belt and the lower belt (carrying the sludge therebetween) converge with one another, applying light pressure to set the thickness and consistency of the sludge prior to reaching the largest perforated drum. Light dewatering or deliquifying continues to occur in the wedge zone as the sludge is compressed between the belts. The degree to which the sludge is compressed corresponds to the angle of convergence of the belts or the wedge angle. The wedge angle may be fixed at all times or mechanically adjusted prior to running the filter press to accommodate a particular sludge. Applicants are aware of no commercial filter press that provides a wedge zone which is continuously adjustable while the filter press is operating to deliquify a material. Wedge zones are shown in U.S. Pat. No. 4,181,616 to Bahr, U.S. Pat. No. 3,894,486 to Sparowitz, U.S. Pat. No. 4,053,419 to Pav, U.S. Pat. No. 4,584,936 to Crandall and U.S. Pat. No. 4,681,033 to Crandall.
The advantages obtained by employing a wedge zone depend upon the extent to which the sludge can be deliquified and preset in a particular wedge zone, and thus relate to the amount of pressure that can be applied to the sludge in the wedge zone. If too much pressure is applied in the wedge zone, the wedge angle being very small, rapid compression of the sludge occurs, resulting in the migration of the sludge beyond the marginal edges of the belts. If this occurs, very little stable sludge will be delivered to the first perforated drum. On the other hand, if the pressure in the wedge zone is too low, the wedge angle being too large, not enough deliquifying occurs in the wedge zone, and the sludge is not fully and evenly distributed across the belts and is too wet when it reaches the first perforated drum. This results in the migration of the wet sludge from the sides of the perforated drum.
Therefore, there is a need for an apparatus for adjusting, in a controlled manner, the wedge angle and thus the wedge pressure in the wedge zone during operation of the belt filter so that a sludge can be efficiently deliquified and properly preset prior to entering the high pressure stage.
Another way in which a sludge can be more efficiently deliquified in a wedge zone is to solve the problem of migrating sludge beyond the marginal edges of the belts. If the marginal edges of the belts are substantially sealed, the sludge can bear a greater wedge pressure without migrating through the substantially sealed marginal edges. Thus, there is also a need to provide practical sealing means for the marginal edges of the belts.
Still further, while known wedge zones increase the efficiency of the deliquification of the sludge, improvements in the inducement of shear forces in the sludge would increase deliquification and presetting in the wedge zone. Thus, any improvements in the deliquifying and presetting action in the wedge zone or elsewhere in a filter press would be accepted and employed immediately.
The present invention is directed to the controlled adjustment of the wedge pressure in the wedge zone, the sealing of the marginal edges of the belts, as well as other techniques and structures for inducing the deliquifying and presetting action in the wedge zone or otherwise increasing efficiency in the wedge zone.